Careful measurement of light scattered from molecules and from small particles in solution is a convenient and versatile laboratory technique for determining their various physical characteristics, such as molecular weights and sizes and, similarly, particle size and structure. Combined with various chromatographic separation devices and concentration detectors, such light scattering measurements also permit the deduction of the differential distributions of these quantities. A detailed review of light scattering measurements, requirements for their performance, and results that may be derived are found in the review article by Wyatt cited above. Because laboratory space is limited and expensive, compact instrument designs are favored. The need for compact design makes the use of the very small semiconductor diode laser light source more desirable than the larger gas laser light source. However, the semiconductor diode laser presents a number of problems that must be overcome for practical use in light scattering instruments and detectors.
Among these problems is the sudden change of power level due to mode-hopping. Small changes in the operating conditions of the laser can produce sudden changes in the operating power of the laser as the dominant oscillation moves from one mode to another. This effect can be caused by small changes in temperature, drive current, or light reflected back into the laser. Changes in reflected light are a natural part of the light scattering measurement since the introduction of a sample causes additional light to be scattered back into the laser and the changing refractive index of the solution also varies the phase of the scattered light. Additional light may be reflected into the laser from the regions of the sample cell where the laser beam enters and exits. Preventing this scattered and reflected light from interacting with the laser requires expensive additional components. For extremely complex sample holding structures such as the flow cell described in U.S. Pat. No. 4,616,927, the large number of parallel surfaces mounted perpendicularly to the incident laser beam renders the complete removal of back reflected components almost impossible. When the temperature and drive current are at the critical point for a mode shift, small random changes in the reflected light level and phase can cause the laser power to fluctuate rapidly up and down by several percent.
The essential physical property measured by a light scattering instrument, or more commonly a light scattering photometer, is the ratio of the light power incident on the sample per unit area to the light power scattered by the sample per steradian. If the laser light source were perfectly stable, the incident power need be measured only once, as a calibration procedure. Unfortunately, the laser power tends to change with temperature, reflected light, drive current, and age. The conventional approach for dealing with changes in laser power is to split off part of the beam and use it to monitor the laser power with an optical detector. The laser monitor signal thus produced may be used either to stabilize the laser power, by providing feedback for an electronic circuit that will adjust the drive current, or to normalize the scattered light signal during mathematical processing of the measured data. In either case, the effect is to divide the scattered light signal by the laser monitor signal. Both of these approaches, however, can still result in small errors. Laser mode-hopping may occur very rapidly, resulting in a change of laser power for a short time, until the monitor feedback signal and control circuitry can adjust the drive current to restore the desired power level. Similarly, a lack of simultaneity or signal averaging symmetry in the measurement of the scattered light detector signal and the laser monitor detector signal can result in significant error if the signals change too rapidly for the monitor to follow. Another source of error is that the laser beam may consist of more than one spatial and temporal component, or beamlet. The laser monitor may respond to a different combination of these beamlets than do the light scattering detectors, thus preventing the monitor from accurately tracking a signal proportional to the scattering signals. For all these masons, it is desirable to avoid the sudden, often high frequency, laser power changes that are caused by mode-hopping.
Diode laser mode hopping noise in optical and communication signal applications occurs at very high frequencies and has been the subject of many research reports and several patents. This very high frequency noise is particularly wordsome in communications applications since it occurs at frequencies comparable to the desired communications signal frequencies themselves. For the case of light scattering measurements generally performed at very low frequencies, it is only necessary for the average scattering detector signal and the average monitor signal to track accurately. The reduction of high frequency mode hopping noise by the use of a 2.5 GHz drive current modulation was reported in the reference by J. Vanderwall and J. Blackburn cited earlier. Also as cited earlier, the reduction of noise in a video disk system by drive modulation at over 100 MHz was reported by Hitachi engineers M. Ojima and S. Yonezawa. IBM engineers K. Stubkjaer and M. Small reported noise reduction using 50 to 200 MHz modulation. Further cited work by Ojima et al. used modulation at 200 MHz to 1 GHz. The optimum modulation drive frequency for high frequency noise reduction was shown to be related to the time delay of light reflected back into the laser, in the cited work by E. Gage and S. Beckens. Modulation drive frequencies of 100 to 450 MHz were studied. In addition, these studies used very high levels of modulation, actually running the drive current below threshold during part of the modulation cycle, and, presumably, shutting off coherent emissions during that period.
In the cited U.S. patent by Ryoichi Ito, assigned to Hitachi, U.S. Pat. No. 3,815,045 (1974), "Method of and Device for Modulating Directly a Semiconductor Laser", the use of a modulated drive current to shift a semiconductor laser between two spatially distinct modes is described, which results in a high frequency modulation of the output beam by optically selecting only one of the spatial modes. This method is not applicable to light scattering photometers because light scattering photometers require stabilized light beams, rather than modulated light beams. The most satisfactory lasers for light scattering instruments operate in only one spatial mode in order to ensure a collimated beam. Operation in other spatial modes as described by Ito would exacerbate stray light problems.
Japanese patent application SHO 59 9086 and its corresponding German application DE 41 33 772 A1 (May 21, 1992) by M. Kohno and J. Itami, assigned to Mitsubishi, describe a particular compact disc reading detector structure in which reflected light causes noise which is best reduced by modulation at 500 to 600 MHz. The optimal frequency selected is based on the distance of the laser source from the reflecting region of the disc. This concept is not applicable to light scattering instruments for several reasons: First, the use of 500 MHz drive current is expensive and inconvenient. Second, because the light scattering sample material is often carried along or through the laser beam by fluid flow, there is no fixed scattering distance, and therefore no single optimum high frequency of modulation. Third, very high frequency noise reduction is of little importance in a light scattering instrument where signal averaging of 0.1 to 10 seconds is typically used.
It is important to note that in all of the prior art, the noise reduction method has focused on the laser output power itself, and does not directly utilize filtering, signal averaging of the detected signal or mathematical ratioing of the signal and laser monitor. Because of our concern with the over-all performance of the whole light scattering instrument which need determine only the ratio of scattered light to incident laser power, it is possible to allow, and even encourage fluctuations in the laser output if the fluctuations can be effectively tracked or removed after signal detection. Our approach to the problem is to shift the frequency of the mode hopping noise above the passband of the signal processing filters in the detection system. Thus the laser drive modulation, matched signal averaging filters, and the ratioing of scattering to incident light are all essential parts of this invention.